Which of the following is a contractile protein?

Which of the following are the special properties of muscles?

Consider the following four statements $(a-d)$ and select the option which includes all the incorrect ones:
$(a)$ Skeletal muscle fibre is a syncytium as the sarcoplasm contains many nuclei.
$(b)$ In the centre of each $I$-band is an elastic fibre called $H$-line which bisects it.
$(c)$ In the resting state,a subunit of troponin masks the active binding sites for myosin on the actin filaments.
$(d)$ The contraction of a muscle fibre takes place by the sliding of the thick filaments over the thin filaments.

Bone-dissolving cells are known as..........

Provide the full names for the following abbreviations:
$(1)$ $HMM$
$(2)$ $ATP$

Sliding filament theory can be best explained as